Microporous membrane materials which are waterproof but permeable to water vapor have been used increasingly for the manufacture of clothing articles over the past few years. In view of the vapor-permeability thereof, one obtains pleasant wearing properties. Such membrane material consists for example of porous polytetrafluoroethylene (hereinafter PTFE).
In more recent times, such membrane material has also been used for the manufacture of waterproof footwear. In this respect, the upper is lined at least on the inside with such membrane material which is preferably designated as a functional layer, U.S. Pat. No. 4,599,810 describes such a footwear article.
Problems arise at the seams in the location where the shoe upper is sewn to the insole. At the stitch locations, the functional layer is perforated and becomes permeable to water.
To overcome this problem, sock-like inserts and inner linings have been employed between the upper and the insole. This sock-like insert may acquire a given shape by welding and thus it need not have seam holes. However, this method involves a high manufacturing expenditure to adapt the sock-like inserts to the particular shoe form.
Another known method is the use of injection-molded rubber or plastic, which in general forms the outer sole around the bottom side of the insole and the lower portion of the upper that is lined with the functional layer and sewn to the insole. In doing so, the connecting seam between the insole and the upper is enclosed by the rubber or plastic sole, i.e. by waterproof material.
Further, the upper usually consists of leather or a textile fabric, made, for example from synthetic fibers.
The molded-on plastic sole provides a seal for the seam between the insole and the upper lined with the functional layer against direct impingement of water. However, the upper materials mentioned particularly leather, direct water in the longitudinal extension thereof. This directing of water involves capillary action. Thus, when the upper portion that is not covered by the plastic sole becomes wet, the water creeps along the upper due to this longitudinal directing effect, and passes to the seam located within the molded-on plastics sole, where the water may then penetrate the functional layer at the seam holes. The functional layer is usually located within a laminate, coated with a protective textile material on the side facing the upper and with a lining material on the side facing the inside. This lining material usually is absorbent and water-conductive. The water that enters along the upper and penetrates through the seam then creeps along the lining into the interior of the shoe. Measurements show that it takes about 10 minutes after the footwear has become wet on the outside for the inner lining to become wet.